In order to promote the use of hydrogen as an energy source, there is a need for a low-cost method for supplying hydrogen that is capable of flexible adaptation to changes in the amount of hydrogen to be supplied. One such hydrogen supplying method being considered is an energy carrier system, where a material containing large amounts of hydrogen, such as ammonia, is transported and stored as a hydrogen carrier, and from which hydrogen is generated in accordance with the time of consumption and consumption amount.
A typical apparatus that uses hydrogen as fuel is the fuel cell. Fuel cells require hydrogen of a high purity to run, and the standard for hydrogen purity for fuel cells is currently defined as 99.97% in ISO 14687-2. If hydrogen of a high purity could be efficiently supplied using energy carrier systems, a greater popularization of fuel cells could be expected.
An example of a known method for generating hydrogen is steam reforming of hydrocarbon gas, such as methane. However, steam reforming requires processing at high temperatures using expensive catalysts such as nickel, which makes the production apparatus expensive as a whole. Moreover, when the mole ratio of steam to the carbon contained in the hydrocarbon used as raw material becomes low, coking of the carbon on the catalyst occurs, which deactivates the catalyst. The production conditions must therefore be carefully controlled corresponding to the amount of hydrogen to be produced. Another known method of producing hydrogen is a catalysis method using a precious metal catalyst such as ruthenium to decompose a raw material such as ammonia at a temperature of 400° C. or higher. However, such catalysis methods have a low decomposition rate of the ammonia, and cannot generate hydrogen pure enough for use in fuel cells at a high yield.
In addition, methods for transforming source gas into plasma to generate and separate hydrogen are being considered. Patent Document 1 discloses a hydrogen producing apparatus including a plasma reactor into which a gaseous raw material is introduced, and a nearly cylindrical hydrogen separating/transporting section for separating hydrogen in the plasma reactor and transporting the obtained hydrogen to the outside of the plasma reactor. The outer wall of the plasma reactor also serves as an external electrode. The hydrogen separating/transporting section arranged coaxially with the external electrode is composed of a porous internal electrode and a hydrogen separating film with a thickness of a few ten μm to a few hundred μm coated on an internal surface of the internal electrode. Ferroelectric pellets of BaTiO3 are filled between the external electrode and the hydrogen separating/transporting section.
Patent Document 2 discloses a hydrogen generating apparatus including a plasma reactor, a high-voltage electrode, and a grounding electrode. In the hydrogen generating apparatus of Patent Document 2 a hydrogen separation membrane functions as the high-voltage electrode, and hydrogen is generated by causing a dielectric barrier discharge between the hydrogen separation membrane and the grounding electrode under normal temperature and atmospheric conditions to transform the ammonia contained in the supplied gas into plasma.
A characteristic of the hydrogen generating apparatuses using plasma discharge of Patent Documents 1 and 2 was that the required electrical power for transforming source gas in the cylindrical reactor into plasma in a uniform manner increased along with the capacity of the reactor. Larger reactors could therefore actually be less energy efficient than smaller ones, and there was therefore a risk of a reduced hydrogen yield when large-scale production of hydrogen was necessary.